This invention relates to a vacuum pump having a suction nipple valve which includes a closure element coupled with the piston of a cylinder-and-piston assembly. One end of a pressure medium conduit opens into the cylinder of the cylinder-and-piston assembly, while the other end of the conduit has an inlet opening which is opened or closed as a function of the operational state of the pump. The invention further relates to a method of operating a vacuum pump which has a suction nipple valve actuated as a function of the operational state of the pump.
Rotary vacuum pumps are driven by means of sealing liquids, preferably oil, in order to achieve a high final vacuum. At the same time, the oil serves for lubricating the bearings and for cooling the pump. After stopping pumps of the above-outlined type--either by usual deenergization or because of power failure or other operational malfunction--there are risks that the oil rises in the oil receptacle particularly if the latter is under vacuum. Such occurrence causes undesired soiling. In order to avoid these disadvantages, a number of solutions are known which are described in an article by Dieter Knobloch and Heinrich Oehmig entitled "Saugstutzensperre verhindert Olrucksteigen an rotierenden Vakuumpumpen" (Suction Nipple Shutoff Prevents Oil Backup in Rotary Vacuum Pumps), published in the periodical Maschinenmarkt (Wurzburg, Federal Republic of Germany), Issue 79 (1973) 54, pages 1191-1193. As illustrated in FIG. 4 of the article, it is known to provide a vacuum pump with a suction nipple valve of the type described above. The pressure medium is air. The inlet opening of the pressure medium conduit is either closed or open, dependent upon the operational condition of the pump. For this purpose, on the pump shaft a centrifugal switch is mounted, by means of which the inlet opening of the conduit is closed upon the start-up of the pump. In this manner, the opening movement of the closure element of the suction nipple valve is effected. If the vacuum pump, for whatever reason, comes to a standstill, the centrifugal switch opens the inlet opening so that air under atomospheric pressure penetrates into the cylinder containing the piston of the suction nipple valve. This occurrence thus effects a closing motion of the suction nipple valve. Further, a nozzle is provided through which an aeration of the pump chamber occurs subsequent to the closing motion of the closure element of the suction nipple valve.
It is a disadvantage of the above-described solution that the undesired air intake which effects an increase of the pressure in the oil receptacle communicating with the intake nipple is still present because it cannot be entirely avoided that one part of the air which causes the motion of the valve piston penetrates between the piston and the cylinder into the suction chamber during a period when the suction nipple valve has not yet assumed its closed position. Additionally, air penetrates into the suction chamber--and increases thus the air intake--through the nozzle which serves for the aeration of the pump chamber. The air intake could be prevented in the known solutions only by ensuring that there prevails only a very narrow clearance between the piston and the cylinder and by omitting the nozzle which serves for a subsequent aeration of the pump. Such a solution, however, would have not only the disadvantage that an automatic aeration of the pump chamber upon reaching standstill no longer occurs but also, that the cylinder-and-piston assembly is very sensitive against soiling and must be made with extremely small tolerances. If, for example, a soiling by oil occurs which is often the case in rotary vane type pumps, then the suction nipple valve no longer operates reliably. The opening period will be significantly lengthened; in case of small pressure differences the valve will not open at all. It is a further disadvantage of decreasing the play that the manufacturing costs increase significantly.